In general, a light emitting diode (LED), which is a light source for an LED lamp, is a semiconductor configured to be driven by current. A current source is required in order to light the LED.
An alternating current (AC)-direct current (DC) converter system is well known as a system for driving such an LED lamp. However, the AC-DC converter system is a switching mode power supply system, the unit cost of which is about 25% the price of a product. That is, the AC-DC converter system is very expensive, which impedes the popularization of LED lamps. In order to solve this problem, an AC direct connection type LED lamp driving device, the price of which is extremely low, has been proposed as alternative technology.
FIG. 1 is a block diagram showing a general alternating current (AC) direct connection driving-type LED lamp driving device according to the conventional art.
As shown in FIG. 1, the general AC direct connection driving-type LED lamp driving device according to the conventional art includes a bridge diode 110 for converting AC input voltage from an AC input power source AC into full-wave rectified voltage, a plurality of LED lamps L1, . . . , and L7, which are loads that are configured to be lit by full-wave rectified voltage, which is output from the bridge diode 110, a first switch LW1 to a seventh switch LW7 for sequentially or non-sequentially driving the LED lamps L1, . . . , and L7 when the LED lamps L1, . . . , and L7 reach operating threshold voltages, and a switching controller 120 for controlling the switches LW1, . . . , and LW7 and a current source CS1.
Each of a first LED lamp L1, a second LED lamp L2, . . . , and a seventh LED lamp L7, which constitute the LED lamps L1, . . . , and L7, may be a single high-voltage LED lamp or a group of LEDs (an LED group).
Reference symbol CS1 indicates a current source CS1 for controlling input current and output current in the AC direct connection driving-type LED lamp driving device shown in FIG. 1.
The LED lamp driving operation that is performed by the general AC direct connection driving-type LED lamp driving device according to the conventional art having the above-stated construction will be described briefly.
When the instantaneous value of AC input voltage is higher than the operating threshold voltage VF of each LEP lamp as the result of an increase in the AC input voltage, the first LED lamp and the second LED lamp are sequentially lit.
That is, when AC input voltage that has been rectified increases from zero voltage and reaches the threshold voltage LED1 VF of the first LED lamp L1, the first LED operating switch LW1 is switched on, with the result that the first LED lamp L1 is lit. When the voltage continuously increases and reaches the threshold voltage LED2 VF of the second LED lamp L2, the second LED operating switch LW2 is switched on, with the result that the second LED lamp L2 is lit. When the voltage further continuously increases, the remaining switches are sequentially switched on, with the result that the remaining LED lamps are sequentially lit. In this way, all of the LED lamps are lit.
When the input voltage decreases after a phase of 90 degrees, the LED lamps are turned off one after another in the order that is reverse to the lighting sequence (in a non-sequential manner) or in the order that is the same as the lighting sequence (in a sequential manner).
Consequently, all of the LED lamps are lit at the rising time and the falling time of the input voltage.
However, the conventional AC direct connection driving-type LED lamp driving device has the following problems.
First, the efficiency of the AC direct connection driving system is basically about 10% lower than that of the SMPS system due to the characteristics of driving technology (input voltage variation and LED light-deviation output characteristics are required to be satisfied).
In controlling most AC direct connection driving systems, a plurality of LED lamp control tabs is provided, as shown in FIG. 1. As the number of control tabs increases, therefore, control efficiency is improved. However, the mitigation of a loss in the voltage of the rear-end LED lamps is limited due to the characteristics of driving technology (input voltage variation and LED light-deviation output characteristics are required to be satisfied), as described above, with the result that the efficiency of a general LED driver integrated circuit (IC) for controlling four groups is about 80%.
The input voltage of a conventional AC direct connection type driving circuit (device) varies depending on the nation (for example, 220 VAC in Korea and 260 VAC in Europe). In addition, there is variation in the input voltage. For example, variation in the input voltage in Korea is 10% of 220 VAC, and variation in the input voltage in Europe is 220 to 260 VAC.
When the input voltage varies, the total operating threshold voltages Total LED VF of the LED lamps, which are low fixed values, decrease in proportion to the magnitude of increase in input voltage. In the conventional art, when the input voltage varies, current is controlled to decrease. When the variation in the input voltage is excessive, the input voltage is cut in order to prevent an increase in input power. These operations are performed in order to complement driving reliability. In this case, however, driving power efficiency is greatly reduced. Consequently, the AC input voltage is limited to vary within a narrow variation range (about ±5% in the worst case). As a result, much power is lost, whereby efficiency is seriously reduced.
In addition, variation in input and optical characteristics with respect to variation in the AC input voltage is great and unstable.
Due to the characteristics of the AC direct connection driving system (variation in the input voltage and optical deviation between LED groups), for example, the total operating threshold voltages Total LED VF of the LED lamps must be set to be lower than the maximum voltage value Vmax, which is a value obtained by multiplying the lower limit value of variation in AC input voltage, which is 180 VAC, by √2 such that all of the LED lamps are lit, thereby achieving maximally uniform optical output.
As shown in FIG. 2, therefore, the total operating threshold voltages Total LED VF of the first LED lamp to the seventh LED lamp are set to be lower than the maximum voltage value Vmax. As a result, the loss in the voltage of the rear-end LED lamps is high due to the control characteristics of the AC direct connection driving type driver IC, whereby power efficiency is greatly reduced.
In particular, when the input voltage varies (increases), as shown in FIG. 2, a loss in the voltage of the rear-end LED lamps abruptly increases in proportion to the magnitude of increase in input voltage. As a result, LED lamp power efficiency is abruptly reduced in proportion to the increase in voltage.
This reduces not only efficiency but also greatly affects product reliability due to an increase in the amount of heat that is generated by the LED lighting device.
Consequently, it is required to improve such an inefficient part.
For example, it can be indirectly seen from simple calculation that, when the efficiency of 220 VAC is 80% and the input voltage varies to 264 VAC, 264/220=120%, and therefore the efficiency may be reduced to about 80%*0/8=64%.
Prior Patent Document 1: Korean Patent Application Publication No. 10-2007-0097060 (Oct. 2, 2007)
Prior Patent Document 2: Korean Registered Patent Publication No. 10-0971759 (Jul. 21, 2010)